When providing information such as passenger announcement or audio entertainment service to a user sitting in a seat in an aircraft, a vehicle, or the like where there is a lot of noise, ambient noise at the seat can be a problem. When using an internal space bounded by continuous walls, such as in an aircraft or a vehicle, the usage site is a kind of sealed structure, and if there are noise sources inside and outside the usage site, the noise environment ends up becoming entrenched for the user. Therefore, depending on the level of noise, the noise can become a physical and mental constraint factor to the user, making the usage site less convenient to use. In particular, when providing services to customers in the passenger compartment of an aircraft or the like, if convenience declines, this can seriously impede the quality of service business.
Especially in the case of an aircraft, the main sources of noise are those involving the noise of machinery used for generating thrust in the aircraft, such as propellers and engines, and sounds related to air flow generated by the movement of the aircraft through air layers, such as wind noise produced by the front end of the aircraft and by the wings during flight, so noise inside the aircraft is unpleasant to the passengers, and also hinders voice announcements and the like, so improvement is strongly desired.
The usual way to reduce noise in a sealed compartment has conventionally involved a method that relies on passive attenuation, and a sound insulation material that is acoustically absorbent, such as a barrier material or an absorbent material, is disposed between the sealed structure and the noise source. A high-density barrier material or the like is used as a barrier material, and a sound deadening sheet or the like is used as an absorbent material. Materials that are acoustically absorbent generally have higher density, and high-density materials lead to increased weight. As the weight increases, fuel consumption increases and the cruising distance decreases. Therefore, this diminishes the economic efficiency and functionality of the aircraft. Also, if used as a structural material, the deterioration of function in terms of strength (prone to scratching, etc.) and design (texture and the like) cannot be ignored.
To solve the problem encountered with noise countermeasures involving the above-mentioned passive attenuation section, a method in which a sound wave having a phase opposite to the phase of the noise is generated has generally been employed in the past as a way to reduce noise with an active attenuation section. This method reduces the noise level at or near the source and prevents the noise from propagating to areas where noise reduction is required. A specific application example that has been proposed is a noise reduction device comprising a microphone that picks up sound generated from a noise source, a controller that amplifies the electric signal inputted from the microphone and inverts the phase, and a speaker that converts the electric signal inputted from the controller into sound and transmits this sound (see Patent Literature 1, for example).
Also, in order to design an active noise reduction device, it is necessary first to find the acoustic transfer function from the speaker to the noise control point. To this end, it is common to generate white noise having flat frequency characteristics in the control frequency band from a speaker, pick up this white noise with a microphone provided at the control point, and measure the transfer function thereof. A method has been proposed in which the external noise level is measured and white noise that is higher than the external noise level by a specific amount (such as 10 dB) is generated in order to avoid the influence of external noise (see Patent Literature 2, for example).